Joining two windows doesn't work
quoting from the manual:


  order to merge two window frames, they must be the same dimension in the direction you wish to merge. For example, if you want to combine two window frames that are side-by-side, they must be the same height. If the one on the left is not the same as the one on the right, you will not be able to combine them horizontally. This is so that the combined window space results in a rectangle. The same rule holds for joining two window frames that are stacked on top of one another; they must both have the same width. If the one above is split vertically, you must first merge those two, and then join the bottom one up to the upper one.




read:
https://www.blender.org/manual/getting_started/basics/interface/window_system/arranging_frames.html?highlight=join%20window